NIST Contributes to Discovery of Novel Quantum Spin-Liquid

Gaithersburg, Md.—An international team of researchers including scientists from the National Institute of Standards and Technology (NIST) has found what may be the first known example of a "spin-orbital liquid," a substance in a never-before-seen quantum mechanical state.

Data taken with synchrotron diffraction indicates a short range, honeycomb-based nanostructure, which is the basis for the anomalous magnetism of Ba3CuSb2O9. NCNR neutron scattering data confirmed this structure and provided evidence for the resulting quantum spin liquid.

The discovery, reported May 4, 2012, in the journal Science, has been sought for years by the physics community. Though the team does not posit immediate applications for the material, its properties relate to the same quantum effects that give rise to superconductivity, in which electricity flows through a material with no resistance, and superfluidity, in which a liquid flows across a surface with no friction.

The term "spin liquid" can be deceptive, as it describes a substance that in many ways fits our conventional understanding of a solid. Indeed, the material the team studied looks like a chunk of earth, but at the molecular level, it is made of copper, oxygen, barium and antimony atoms arranged in a crystalline lattice structure. In this particular structure the copper atoms exhibit unusual properties generally associated with liquids. Specifically, their magnetic orientation remains in a constant state of flux.

When materials with magnetic atoms—like iron—solidify, they generally do so in crystal structures whose atoms have an orderly arrangement of magnetic orientations. (When magnetic atoms interact "ferromagnetically" you get a refrigerator magnet.) Because magnetism stems from a quantum property in the atom's electrons called spin, another way of saying this is that the spins in these atoms' electrons all line up in a single direction. Ferromagnets feature an orderly, static arrangement of electron spins.

In the material the team studied, the copper atoms are positioned within the lattice in such a way that their spins incessantly disturb each other, pushing each other around so they become unable to form an ordered configuration, instead creating what is aptly termed a "frustrated" magnet.

"You'd generally expect the copper spins and the corresponding electronic orbitals to become locked into a specific pattern upon cooling, but in this case that doesn't happen," says Collin Broholm, a physicist at both the NIST Center for Neutron Research (NCNR) and the Institute for Quantum Matter at Johns Hopkins University. "While the crystalline lattice is solid, the atomic spins continue to fluctuate. These spin fluctuations embody a fluid aspect of the material, so we end up with a quantum fluid within a solid."

The research team came from several institutions, including the University of Tokyo and Nagoya and Osaka University in Japan, and the University of California, Santa Cruz, in the United States. The material itself was created at the University of Tokyo's Institute for Solid State Physics. NIST's contribution to the discovery was in neutron scattering measurements, which revealed the fluid nature of the spin systems and also provided evidence the material has a different lattice structure than had previously been assigned to it – a structure that allows the copper atoms to affect one another as they do.

"Magnetic neutron scattering gave us a clear indication of some sort of quantum mischief in this compound," Broholm says. "The data show the spins don't develop static long-range order, but instead behave as a magnetic quantum fluid. Separate nuclear diffraction measurements also at the NCNR provided essential new information about the underlying hexagonal structure."

Broholm says the findings expand our understanding of what qualitatively different behaviors are possible in magnetic materials.

"Here we have a new type of magnetism, characterized by the lack of static orbital and spin order at low temperatures," he says. "Instead what we see is orbital and spin quantum fluctuations. This could provide new opportunities in materials science and engineering down the road, but for now we're excited to have encountered a qualitatively new state of magnetism where spin and orbital quantum fluctuations prevail."

The NIST findings were among the first to be made with the NCNR's multi-axis crystal spectrometer (MACS), which is supported in part by the National Science Foundation.